The present invention generally relates to a heat responsive coating and a coating composition, and particularly, to a coating and coating composition which allow high speed marking of substrates by the use of a laser beam.
Laser marking technology has been used to mark substrates. For example, laser marking technology makes use of laser energy focused to the surface of the substrate. Carbon dioxide lasers having a wavelength in the infrared region are generally used for this application.
At times, laser marking technology has certain advantages over other marking technologies, e.g., ink jet printing technology. For example, the maintenance of a laser equipment may be easier and more economical in certain circumstances than the maintenance of other types of markers. Since the laser marking technology does not depend on the use of an ink in a liquid state to produce a mark, it is less prone to printing problems caused by ink.
In addition, laser marking technology allows marking of substrates at extremely high speeds. An example of the use of this technology is in the marking of expiration dates on plastic soda bottles. During laser marking, the rate of movement of the conveyor carrying the soda bottles generally ranges from about 100 to about 300 feet per minute, and it can be as high as 500 feet per minute.
Although laser marking technology has advantages, substrates suitable for laser marking are presently lacking. For example, plastics such as white high density polyethylene, metals, and white or corrugated paper cannot be readily marked.
Metal surface is difficult to mark by laser marking technology as most metals have high heat conductivity and/or light reflectance values as well as high melting points. One or more of these characteristics reduce the effectiveness of the laser energy received by the metal surface during marking. In the case of corrugated paper, the laser tends to damage the surface. As regards plastics, the mark is created by the laser by altering, e.g., by melting or degrading, and the resulting surface of the plastic is generally of the same color as the plastic itself. This can lead to an inconspicuous mark.
Thus, there exists a need for substrates or substrate surfaces that are suitable for laser marking. There further exists a need for substrates or surfaces that are suitable for creating a white mark against a dark or colored background.
The advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.